This talk is based on research work on multi-UAV systems (or, drone swarms) at AAU Klagenfurt, performed over approx. a decade in the research focus area 'Networked and Autonomous Systems'. Several research groups from different AAU and associated institutes have been and are still involved in research on multi-UAV systems in various funded projects; a doctoral school on 'Networked Autonomous Aerial Vehicles (NAV)' started in late 2017.We will initially cover the targeted (civilian-only) application areas of systems of multiple UAVs, ranging from first-responder support to delivery services, as well as the research topics that we addressed. The major components of multi-UAV systems -- UAV platform, sensing, coordination, and communication -- will be introduced and some of our contributions will be highlighted. The focus of the talk will be on the significant challenges, our research work and experiments, and lessons learned on (air-to-air and air-to-ground) communication in multi-UAV networks. The objectives and the ongoing (communication) research in the NAV doctoral school on autonomous 3D reconstruction and navigation will be presented, including a novel model to evaluate the utility of information distribution schemes. Our initial steps into utilizing 5G (currently, LTE-A) for multi-UAV communication, including edge computing support, will be briefly discussed. Whenever appropriate, we will address why and how we use adaptation (mostly, of payload data) in order to cope with network constraints.